There : https://bdsc.webapps.blackberry.com/...et20/register/But please be aware of what you're doing. This is beta, this has limitations, you may loose stored information ... in brief, read the forum and OS2 related posts before upgrading.Anyway: backup your whole PB prior to anything (and if you've never done this before, well, even if not upgrading, it's still a good idea !)

I can imagine this has been covered many-a time on here, but struggling to find anything on it.

Is there a way I can sign up as a developer so I can download OS2 for my playbook?

There is no need to "sign up" as a developer because RIM has recently removed all barriers to becoming a PB developer. With this power now in the hands of consumers, you have a responsibility to do your due diligence prior to downloading the developers' beta OS. There are many threads here to guide you as your determine if you really want to do this. Start by reviewing the stickies. Rule #1 is: Backup your system before proceeding any further!